A promoter is generally defined as a nucleic acid sequence upsteam or downstream from a transcribed gene, and to which RNA polymerase must bind if it is to transcribe the flanking gene into messenger RNA. A promoter may consist of a number of different regulatory elements that affect a structural gene operationally associated with the promoter in different ways. For example, a regulatory element may enhance or repress expression of an associated structural gene, subject that gene to developmental regulation, or contribute to the tissue-specific regulation of that gene. Modifications to promoters can make possible optional patterns of gene expression, using recombinant DNA procedures. See, e.g., Old and Primrose, Principles of Gene Manipulation (4th Ed., 1989).
Transgenic plants expressing peptides that inhibit or kill a particular pest or pathogen provide a method for decreasing crop damage and loss. For example, expression of the Bacillus thuringiensis protein in transgenic corn provides resistance to the European corn borer. However, transgene expression in all tissues of a plant (constitutive expression) can be disadvantageous as it can expose non-target organisms to the transgenic protein and can increase the selective pressure for the development of pathogens and pests which are resistant to the transgenic protein. High levels of transgene expression throughout a plant may also negatively affect growth and yield of the plant. An alternative strategy is to express a toxic peptide only in the organ or tissue affected by a particular pest or pathogen. Implementation of this strategy against pests and pathogens that attack plant roots has been hampered by the lack of characterized root-specific promoters.
Transcription of a gene is initiated when a stable complex is formed between RNA polymerase enzyme and a gene promoter. Promoters generally occur at the beginning of all transcription units, are typically about 100 base pairs in length, and generally are located immediately upstream from the start site of transcription. See e.g., Maniatis et al., Science 236:1238 (1987). Promoters vary in their “strength”; that is, in their ability to accurately and efficiently initiate transcription. The RNA polymerase holoenzyme is thought to cover a region of about 50 bases immediately upstream of the transcribed region. In some cases the strength of transcription initiation may be enhanced by auxiliary proteins that bind adjacent to the region of the promoter which is immediately upstream from the transcribed DNA. See, e.g., Singer & Berg, Genes and Genomes, 140–145, University Science Books, Mill Valley, Cailf. (1991).
U.S. Pat. No. 5,459,252 to Conkling and Yamamoto describes the RB7 root promoter.
U.S. Pat. No. 5,837,876 to Conkling, Mendu and Song describes the RD2 root cortex specific promoter, also known as the NtQPT1 promoter.